Coverage

From Adylitica

Coverage is a term for the Impressions share given by Google to your AdWords keywords.

When you buy keywords, Google will only show your ad to a fraction of the people searching for those keywords. This fraction is coverage.

Why Google does this

Google has a variety of reasons for not showing your ad on every keyword, from their goal of keeping overall ad quality high so that people don't tune the ads out, to promoting the value of position #1 by artificially constricting the impressions of lower position ads.

Optimization

Improving coverage is related to quality score and bid price, the higher those are, the more coverage you will receive.

Coverage is an invisible quantity, Google does not report what coverage percentage you are receiving. You can however use the Google Keyword Tool to try and estimate the number of searches for keywords, and compare that against the number of impressions you are receiving.